This invention relates generally to jet aircraft exhaust nozzles and more particularly, to methods and apparatus for adjusting a nozzle throat within a jet aircraft exhaust nozzle.
At least some known engines include either a fixed exhaust nozzle system, such as is typical of commercial subsonic engines, or a variable exhaust nozzle system, such as is typical of supersonic military aircraft. The geometry of fixed nozzle systems are not kinematically changed or variable and as such may not operate as efficiently as variable exhaust nozzle systems.
More specifically, variable geometry systems are configured to operate over a wide range of pressure ratios (P8/Pamb) by adjusting a nozzle throat (A8) based on the demands of the engine cycle, and adjusting a nozzle area ratio (A9/A8) to facilitate achieving a desired engine performance at various operating points.
In at least some known variable exhaust nozzle systems, A8 and A9/A8 control is established by “linking” A9/A8 to A8, i.e. establishing a kinematically-linked area ratio schedule. For example, at least one known engine includes a variable exhaust nozzle system that includes a circumferential series of overlapping flaps and seals that define a convergent flowpath that establishes a desired nozzle throat A8. A similar set of overlapping flaps and seals is connected to an aft end of the convergent flaps and seals and establishes a divergent portion, or an exit area (A9) of the nozzle. The divergent flaps are also kinematically-linked using a separate kinematic member, such as a compression link that is coupled to a relatively stationary part of the exhaust system, such as a duct. The resulting four bar linkage, duct, convergent flap, divergent flap, and compression link, define the kinematic relationship of the exit area A9 to the nozzle throat area A8, and thus also defines the A9/A8 schedule as a function of A8. Such an arrangement typically results in an A9/A8 schedule which increases as A8 increases.
However, the use of an overlapping flap and seal structure in the nozzle design may result in numerous leakage paths which may cause a corresponding decrease in engine operating efficiency. Additionally, the relatively large quantity of parts used to fabricate the nozzle may increase the cost, weight, and maintenance of such engines.